Obj = function(i,newFn) {
   this.i = i;
   if( newFn ) this.fn = newFn;
}
Obj.prototype.fn = function(a) { return a+this.i; }

o1 = new Obj( 3, function(a) { if( a<0 ) return 0; return Obj.prototype.fn.call(this,a) } );
